In a three phase alternating current (AC) system, knowledge of the phase voltages is often required for various control purposes. Because a neutral line is rarely available, the phase voltage is often defined with respect to a virtual neutral point. This virtual neutral point is usually established as the intersection of three equal impedances coupled to respective ones of the voltage lines.
In practice, inverting operational amplifiers (“opamps”) may be employed to provide an estimate of a scaled value of each phase voltage. The positive input of each of the opamps is referenced to a signal (isolated) ground. In power converters, this signal ground is often the converter ground bus. Unfortunately, a common mode voltage can develop between the virtual neutral point and the signal ground. This common mode voltage can grow to the point that one or more of the opamps fails to indicate accurate line voltages.
The common mode voltage may be reduced by placing an impedance between the virtual neutral point and the signal ground. However, this distorts the virtual neutral point, and the outputs of the inverting amplifiers may no longer accurately reflect the phase voltages.
To resolve a large common mode voltage swing in a switching environment, an isolation transformer or an opto-coupler may be employed with the opamps. The transformer can provide excellent isolation and good linearity, but often its size is a problem as requirements for power converters dictate smaller footprints and higher power density. While opto-couplers provide smaller size, the non-linearity inherent in opto-couplers degrades the accuracy of the voltage measurement. Furthermore, the added isolation may require an isolated bias supply for the power converter, posing additional cost or surface area requirements. Other solutions may work well for a symmetric or balanced voltage condition. However, these solutions usually do not accommodate line voltages that are asymmetric or unbalanced, which is more often the case in real-world environments.
Accordingly, what is needed in the art is an improved way to provide phase voltages from voltage lines that lack a neutral line and that accommodates voltage imbalances.